Structured substrates for optical surface profiling

ABSTRACT

This disclosure provides methods and devices for the label-free detection of target molecules of interest. The principles of the disclosure are particularly applicable to the detection of biological molecules (e.g., DNA, RNA, and protein) using standard SiO2-based microarray technology.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/912,565, filed Oct. 25, 2007, which is a 35 U.S.C. § 371 nationalphase entry of international application PCT/US2006/015566, withinternational filing date Apr. 25, 2006, which claims the benefit of andpriority to U.S. Provisional Patent Application Ser. No. 60/674,642,filed Apr. 25, 2005, the entire contents of each of which areincorporated by reference herein in their entirety.

FIELD OF THE INVENTION

This present invention relates to optical detection systems forbiological and other molecules.

BACKGROUND OF THE INVENTION

Fields as diverse as bioresearch, ecology, medicine, pharmacology, drugdiscovery and biohazard detection have a critical need for large-scalebiomolecule affinity sensing. Affinity sensing detects the presenceand/or affinity of “target” biomolecules using other “capture”biomolecules. Target molecules include DNA segments, RNA segments,protein, and small molecules and the capture molecules are oftenderivatives of their natural (i.e., in vivo) binding partners which arealso typically DNA, RNA, and protein, but may also include smallmolecules having a high binding affinity. Microarray detection systemsare a technology that has the potential to assess large numbers oftarget molecules in a relatively rapid manner, making it a usefulhigh-throughput methodology (FIG. 1). From tens to more than a milliondifferent capturing agents are fixed in localized spots or features onthe microarray substrate (e.g., glass). Typically, target molecules areobtained from a biological or environmental sample, purified, andfluorescently labeled with a dye molecule including but not limited toCy5, Cy3, quantum dots, or biotin for subsequent labeling withstreptavidin or antibodies. The prepared sample is exposed to the array.The array is then rinsed and placed in a fluorescent scanner thatmeasures the fluorescent signal from every target molecule location.

Throughput refers to the number of target molecule spots measured in asingle experiment. Each spot measures the affinity for molecules in thesample for the fixed capturing agent attached to the substrate at thatposition. High throughput methods are advantageous because they allowthe simultaneous detection of a large number of target molecules in asingle experiment.

Microarrays are actively being used today in biological research andtheir use is expected to expand over the next decade in the areas ofmedical diagnosis, drug discovery, and bio-weapons detection. Currently,DNA microarrays observing DNA-DNA interaction constitute the vastmajority of microarray use, although the technology for protein-proteinand protein-DNA arrays is rapidly advancing.

Despite widespread use, microarrays have their limitations.Specifically, the target molecules must be fluorescently labeled fordetection. The labeling may add ambiguity and, in certain instances,precludes microarrays from many applications where affixing a label tothe target molecules may not be practical or possible. Proteins, forinstance, pose a particularly significant labeling challenge becauseattachment of a fluorescent label is likely to alter the conformationand hence the binding properties of the protein. Even DNA labeling, themost widely used and reliable microarray assay, can be unreliablebecause (i) the label may affect the DNA binding properties, (ii) thelabeling procedure may be time consuming and costly, (iii) each DNAmolecule may receive zero, one, or more than one label reducing thestatistical significance of the measured result, (iv) labels maynon-specifically bind to the background and cloud the signal, and/or (v)background auto-fluorescence from the substrate may cloud thefluorescent label signal. Thus, a label-free detection technology formicroarray assays is preferable.

There are a number of optical label-free technologies currently underdevelopment including waveguides [Lukosz 1990], surface plasmonresonance [Brockman 2000], optical gratings [Lin 2002], and cantilevers[Zhang 2004]. None of which these technologies have yet demonstratedsignificant throughput. Surface plasmon resonance (SPR), for example, isavailable with the capability of 400 simultaneous observation sites. SPRrequires the use of a metallic surface which precludes thewell-established and accepted microarray chemistries developed for SiO₂.Also, SPR does have the benefit of detecting binding events in real-timewhich enables the observer to gather kinetics information about thebinding reactions. Likewise, however, standard fluorescent microarraysdemand dry samples which also precludes obtaining real-time kineticsinformation. Thus, SPR and standard fluorescent-based microarray assaysare satisfactory only for those applications in which throughput is morecritical than kinetics information. There is a need for a system thatmay be used either for real-time measurements for kinetics information,or as a dried assay to collect only binding data.

Optical surface profilers are devices that detect small height changesacross a surface using optical interference measurements. Opticalsurface profilers are used in many semiconductor processing labs andwork by one of two principles: phase shift interferometry (PSI) or whitelight interferometry (WLI) (FIG. 2). PSI works by illuminating areflecting sample with single wavelength light. The illumination beam issplit so that part reflects off the sample and part reflects off areference surface before they are recombined and imaged on a camera. Thebeams interfere when combined to form an interference pattern also knownas an interferogram which is imaged and recorded by a camera (e.g., aCCD camera). The reference mirror position is scanned to createdifferent path lengths for the reflected beam while the interferogram ateach position is captured by the camera. When the path length betweenthe reference reflector position and the sample surface position at aparticular location is the same, or off by an integer multiple of thewavelength, the intensity at that pixel is maximum. This indicates therelative surface height at that position. This PSI method works wellwhen the measuring relative heights shorter than the one half of thewavelength (λ/2) to avoid ambiguity in the relative measurement.

WLI is an alternative optical profiling technique that avoids theambiguity inherent in PSI. The WLI setup and measurement procedure isessentially the same as for PSI (i.e., the light is split with partgoing to the sample, part going to a reference reflector, and thereflections are combined and images onto a camera). The differencebetween the techniques is that instead of using a single wavelength(PSI), a spectrally broad illumination source is used. With a spectrallybroad source, the two combined beams will only interfere constructivelywhen the path length for either reflection is the same without theinteger multiple of the wavelength caveat of the PSI method. Theseexisting optical profiling methods in the semiconductor field, however,lack sensitivity for biosensing applications where low-index biomaterialis binding to a low-index glass surface.

Optical interferometric methods have been more recently developedspecifically for biosensing applications [Pichler 1996; Moiseev 2006].One such method is spectral interference (SI). The sample used in SIconsists of target molecules on a semitransparent layer. Lightreflecting from both the top and bottom surfaces of the semitransparentlayer with the biomaterial interferes in the reflected beam (FIG. 3).The spectrum of the combined reflections from the top and bottom layerinterface is used to determine the layer thickness and hence moleculebinding to the substrate surface. Some wavelengths will experienceconstructive interference while other wavelengths will experiencedestructive interference based on the thickness of the semitransparentlayer. There are two primary reflections, one from the top surface andone from the bottom surface with less significant higher orderreflections that make multiple passes through the semitransparent layerand contribute less to the signal. While these secondary reflectionscontribute less, they do improve the measurement by helping distinguishthe wavelengths experiencing constructive interference from wavelengthsexperiencing destructive interference. The combined reflected beam ismeasured by a spectrometer. Wavelengths where the interference isdestructive are attenuated. The spectrum is used to determine at whichwavelengths the interference is destructive and constructive, and hencethe apparent thickness of the semitransparent layer, for which boundbiomaterial increases the apparent thickness. Thus, knowing the initialthickness of the semitransparent layer allows a calculation of theheight of the biomaterial on the surface. The disadvantage of thismethod for use as a biosensor is that the method is performed by aspectrometer and can be applied to only one location at a time. It isdifficult to measure the spectrum of spectrally broad light at manynearby locations simultaneously, meaning that the technique is notuseful for the high resolution imaging necessary to read modernmicroarrays.

The present invention solves many of the problems of the prior art,including providing a real-time, label-free microarray system suitablefor high throughput screening. Additionally, the invention may beadapted to take advantage of the well-established chemistry developedfor attaching capture molecules to SiO₂-based microarray substrates.

SUMMARY OF THE INVENTION

The present invention provides methods and devices for the label-freedetection of target molecules of interest. The principles of thedisclosure are particularly applicable, but not limited to the detectionof biological target molecules (e.g., DNA, RNA, protein, smallmolecules, and environmental contaminants) using standard SiO₂-basedmicroarray technology.

In one aspect, the invention provides a layered substrate comprising abase layer, at least one, coating layer having a refractive indexdifferent from that of the base layer, and a plurality of spatiallydistinct binding locations, wherein each of the binding locationscomprises capture molecules bound to the topmost coating layer.Desirably, the base layer has a high refractive index. In otherembodiments, the at least one coating layer has an index between about1.1 and about 1.7 (e.g., about 1.4). Particularly useful base layerscomprise silicon and particularly useful coating layers comprise SiO₂,Si3N4, and gold. In other desirably embodiments, the layered substratecomprises at least two different coating layers. Optionally, the layeredsubstrate is constructed having a plurality of reaction wells. Further,each well may be considered a binding location.

Desirably, each of the binding locations comprise a single type ofcapture molecule and capture molecules are preferably DNA, RNA, protein,or small molecules having high affinity and specificity for a targetmolecule of interest. In useful embodiments, the capture molecules arecovalently bound to the topmost coating layer. Particularly usefullayered substrates comprise a base layer of Si and a coating layer ofSiO₂, wherein the coating layer has a thickness of between about 150 nmand about 20 microns, but preferably between about 1 micron and about 20microns or between about 150 nm and about 500 nm.

In another aspect, the invention provides an optical detection apparatuscomprising:

-   -   (i) a tunable light source, wherein the light source produces        substantially collimated light characterized by an illumination        wavelength;    -   (ii) a reflective substrate comprising capture molecules bound        to the uppermost surface; and    -   (iii) a photodetector array operably linked to a central        processor capable of measuring the intensity of the light        reflected from the substrate.

In some embodiments, the light source is a tunable laser or a broadspectrum light source operably linked to a tunable filter. In otherembodiments, the substrate is a layered substrate constructed inaccordance with the principles of this disclosure. Useful photodetectorarrays include a CCD camera and an InGaAs array. Desirably, the lightsource is tunable over a wavelength range of at least 5 nm, 15 nm, 50nm, or more. Useful wavelength ranges include, for example, wavelengthswithin the range of about 1450 nm to about 1650 nm. Desirably, thecentral processor is capable of calculating reflectivity curves ofillumination intensity as a function of illumination wavelength forsubstantially every pixel of said photodetector array.

In another aspect, the invention provides an optical detection apparatuscomprising:

-   -   (i) a light source capable of producing an illumination beam,        wherein the illumination beam is substantially collimated;    -   (ii) a reflective substrate comprising capture molecules bound        to the uppermost surface;    -   (iii) a reference reflector having substantially the same        refractive index as the reflective substrate;    -   (iv) a first beam splitter capable of splitting the illumination        beam into a sample illumination beam and a reference        illumination beam, the beam splitter further capable of        directing said sample illumination beam onto a reflective        substrate and directing the reference illumination beam onto a        reference reflector;    -   (v) a second beam splitter capable of combining the light        reflected from the reflective substrate with the light reflected        from the reference reflector into an imaged light beam;    -   (vi) a means for displacing the reference reflector in a        direction of the reference illumination beam; and    -   (vii) a photodetector array positioned to measure the intensity        of the imaged light beam operably linked to a central processor        capable of capturing images produced by the photodetector array.

The illumination beam contains either substantially a single wavelength(e.g., generated by a single wavelength laser and a broad spectrum lightsource operably linked to a narrow bandpass filter) or multiplewavelengths (e.g., white light). In other embodiments, the substrate isa layered substrate constructed in accordance with the principles ofthis disclosure. Useful photodetector arrays include a CCD camera and anInGaAs array. Desirably, the light source is tunable over a wavelengthrange of at least 5 nm, 15 nm, 50 nm, or more. Useful wavelength rangesinclude, for example, wavelengths the range of about 1450 nm to about1650 nm. In one embodiment, the first beam splitter and the second beamsplitter are combined in a single element. Desirably, the centralprocessor is capable of calculating reflectivity curves of illuminationintensity as a function of the displacement position of said referencereflector for substantially every pixel of said photodetector array.

In another aspect, the invention provides a method for measuring targetmolecule binding to a microarray, the method comprising:

-   -   (i) providing a microarray comprising a plurality of spatially        distinct binding locations, wherein each binding location        comprises substantially a single type of capture molecule bound        to the surface of said microarray and each type said capture        molecules specifically bind a type of target molecule;    -   (ii) contacting the microarray with a sample comprising one or        more target molecules;    -   (iii) assessing the binding of said target molecules to said        capture molecules by:        -   (a) sequentially illuminating the microarray with varying            wavelengths using a tunable light source;        -   (b) measuring the intensity of the light reflected of the            microarray at each of the illuminating wavelengths using a            photodetector array;        -   (c) calculating the substrate reflectivity as a function of            illuminating wavelength for each pixel of the photodetector            array; and        -   (d) comparing the calculated function to the function            calculated prior to the contacting step (ii), wherein a            difference in the function at a pixel is an index of the            binding of the target molecules to the capture molecules.

Optionally, the microarray comprises a layered substrate constructed inaccordance with the principles of this disclosure. Useful capturemolecules include, for example, DNA, RNA, protein, and small moleculesand are preferably covalently bound to the uppermost surface of themicroarray. The target molecules may be present in any type of sample,but particularly useful samples include biological and environmentalsamples. Useful photodetector arrays include a CCD camera and an InGaAsarray. Desirably, the tunable light source is tunable over a wavelengthrange of at least 5 nm, 15 nm, 50 nm, or more. Useful wavelength rangesinclude, for example, wavelengths the range of about 1450 nm to about1650 nm.

In useful embodiments, the comparing step (d) comprises assessing eitherthe difference or the ratio between the function and the function priorto the contacting step (ii).

The foregoing method may also be adapted for use wherein the microarrayis present within a flow cell and the assessing step (iii) is repeatedat least three times. In this case, it is desirable that the assessingstep (iii) is performed at least once prior to binding equilibriumbetween at least one of the target molecules and one of the capturemolecules.

In another aspect, the invention provides a method for measuring targetmolecule binding to a microarray, the method comprising:

-   -   (i) providing a microarray comprising a plurality of spatially        distinct binding locations, wherein each binding location        comprises substantially a single type of capture molecule bound        to the surface of said microarray and each type the capture        molecules specifically bind a type of target molecule;    -   (ii) contacting the microarray with a sample comprising one or        more target molecules;    -   (iii) assessing the binding of the target molecules to the        capture molecules by:        -   (a) providing an illumination beam;        -   (b) splitting the illumination beam into a sample            illumination beam that is directed onto said microarray, and            a reference illumination beam that is directed onto a            reference reflector;        -   (c) combining the light reflected from said microarray with            the light reflected from the reference reflector into a            reflected beam;        -   (d) measuring the intensity of the reflected beam using a            photodetector array;        -   (e) repeating steps (b)-(d) for a plurality of reference            reflector positions;        -   (f) calculating the reflected beam illumination intensity as            a function of the reference reflector position for each            pixel of the photodetector array; and        -   (g) comparing the calculated function to the function            calculated prior to the contacting step (ii), wherein a            difference in the function at a pixel is an index of the            binding of the target molecules to the capture molecules.

Optionally, the microarray comprises a layered substrate constructed inaccordance with the principles of this disclosure. Useful capturemolecules include, for example, DNA, RNA, protein, and small moleculesand are preferably covalently bound to the uppermost surface of themicroarray. The target molecules may be present in any type of sample,but particularly useful samples include biological and environmentalsamples. Useful photodetector arrays include a CCD camera an InGaAsarray. The illumination beam is substantially a single wavelength (e.g.,generated either by a single wavelength laser or a broad spectrum lightsource operably linked to a narrow bandpass filter). Alternatively, theillumination beam comprises multiple wavelengths (e.g., white light).Useful wavelength ranges for the illumination beam include, for example,wavelengths the range of about 1450 nm to about 1650 nm.

In useful embodiments, the comparing step (g) comprises assessing eitherthe difference or the ratio between the function and the function priorto the contacting step (ii).

The foregoing method may also be adapted for use wherein the microarrayis present within a flow cell and the assessing step (iii) is repeatedat least three times. In this case, it is desirable that the assessingstep (iii) is performed at least once prior to binding equilibriumbetween at least one of the target molecules and one of the capturemolecules.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing and other features and advantages of the presentdisclosure will be more fully understood from the following detaileddescription of the exemplary embodiments, taken in conjunction with theaccompanying drawings in which:

FIG. 1 is schematic of a microarray methodology using fluorescencedetection;

FIG. 2 is a schematic diagram for a device capable of performing phaseshift interferometry (PSI) or white light interferometry (WLI);

FIG. 3 is schematic diagram for a device capable of measuring spectralinterference (SI); the inset is a hypothetical interference spectrumthat could be obtained using SI;

FIG. 4A is schematic diagram of the direct reflectivity methodology; theinset is a simplified schematic showing the light paths;

FIG. 4B is a schematic diagram showing features the layered substrate(e.g., microarray) and parts of the direct reflectivity methodology;

FIGS. 5A-B are examples of the data expected to be measured using adirect reflectivity methodology;

FIG. 6 is a schematic diagram of the split beam interferometrymethodology; the inset is a simplified schematic showing the lightpaths;

FIGS. 7A-B are examples of data expected to be measured using a splitbeam interferometry methodology;

FIGS. 8A-B are examples of data expected to be measured using a modifiedsplit beam interferometry methodology;

FIG. 9 is a schematic diagram of the direct reflectivity methodology andlayered substrate adapted for use in a flow cell and configured toobtain real-time binding data;

FIG. 10 is a schematic of the optical component set-up used in aproof-of-concept experiment for the direct reflectivity methodology;

FIGS. 11A-C show the data obtained from a proof-of-concept experimentfor the direct reflectivity methodology; FIG. 11A is a graph showing themeasured reflective intensity as a function of wavelength for pixelscorresponding to physical locations having a height difference of 10 nm;FIG. 11B is a 3D mesh graph of the surface of a test substrate; FIG. 11Cis a 2D image of pixel intensity showing the apparent pixel heightcorresponding to the surface of a test substrate;

FIG. 12 is a schematic optical detection apparatus illustrating the useof a layered substrate containing reaction wells.

DETAILED DESCRIPTION

The present invention provides devices and methods for opticallyprofiling the height of a substrate. These techniques are applied to thedetection of target molecules bound to the surface of microarrays.Significant advantages over prior art methods include, but are notlimited to, the label-free detection of biological and environmentaltarget molecules in a microarray-style assay (i.e., using capturemolecules to immobilize the target molecules) that allows for highthroughput screening. Further, the invention may be adapted to providereal-time binding information in order that binding kinetics ofindividual target-capture molecule interactions may be determined.

Three interferometry methodologies consistent with the principles ofthis invention are described. In each case, the highly sensitivedetection to small height changes of a low-index binding surface isenabled by one or more semitransparent layers below the binding surface.When adapted to microarray detection, the low-index binding materialconsists of the low-index substrate with immobilized capture moleculeson the surface. Captured target molecules (i.e., the molecules ofinterest) causes an increase in the apparent height of the bindingsurface, where the height change is an indicator of the amount of targetmolecules bound to the surface.

A. Direct Reflectivity Method

The direct reflectivity method is the primary substrate enhanced methodfor the label-free detection of microarray binding. As described in moredetail below, the basic principle of the invention uses a topillumination source bright field microscope which is reflected from thesurface into a photodetector array (e.g., a CCD camera). The image thatis formed is essentially an image of the reflectivity of the sample atevery point. It would be difficult to detect molecules binding to themicroarray surface fabricated on standard glass and using a normalmicroscope and collimated illumination source because the reflectivitychange resulting from target molecule binding would be too small toreliably quantify.

Several modifications must be made to the standard microscope andmicroarray set-up in order to obtain significant and observable changesin reflectivity caused by target molecule binding. FIG. 4A and 4B areschematic illustrations of one such set-up that illustrates theprinciples of the direct reflectivity method. The illumination source110 is a tunable laser and an appropriate photodetector array 130(camera) is selected based on the laser's wavelength range. Theillumination beam 112 is directed from above onto a layered microarray120, preferably perpendicular to the plane of the microarray 120.Optionally, the illumination beam 112 is directed onto the microarray120 using a beam splitter 140. The imaged light 114 is reflected fromthe microarray 120 onto the photodetector 130 where an image iscaptured. Optionally, objectives 150 are used to focus the illuminationbeam 112 and/or the imaged light 119. Desirably, all optical componentsare coated with an anti-reflective coating appropriate for thewavelength range used. The microarray 120 is fabricated as a layeredsubstrate consisting of a thick, reflective lower substrate (base layer121) and a thin coating of a low index material (coating layer 122). Forexample, one particularly useful microarray substrate has 10 microns ofSiO₂ layered on a 300 micron thick Si wafer. FIG. 4B shows a simplifiedschematic diagram of the principles of this embodiment. The microarray120 is formatted as a plurality of spatially distinct locations 129. Thelocations 129 may have the same or different target molecule 124 bindingspecificities compared to each other location 129 on the microarray 120.Each location 129 contains a plurality of substantially identicalcapture molecules 123 that specifically bind a single type of targetmolecule 123. To measure target molecule 124 binding to the surface ofthe microarray 120, the laser 110 wavelength is then swept through itstuning range and the reflectivity from each position on the microarray120 is recorded as a function of position and wavelength, resulting in areflectivity vs. wavelength curve. As target molecules 124 bind to thecapture molecules 123 on the microarray 120 surface, the reflectivityvs. wavelength curve 132 for each point of the surface will change in anobservable way compared to the unbound state.

In one example of the direct reflectivity method, the tunable laserillumination source 110 has a range of 1500 nm to 1580 nm. Thephotodetector array 130 is an InGaAs array based camera appropriate forthese wavelengths. The layered microarray 120 is fabricated as a 10micron SiO₂ coating layer 122 on 300 micron base layer 121 of Si. As thelaser wavelength (illumination light 112) is tuned over the 80 nm range,the resulting reflectivity vs. wavelength curve 132 is characterizedboth a maximum and a minimum value (FIG. 5A). Small amounts of targetmolecule 24 binding to the microarray 120 will shift the wavelength ofmaximum and minimum reflectivity (FIG. 5A). The photodetector array 130records these curves for each pixel as the wavelength is tuned, and acentral processor (i.e., computer) compares how each pixel is shiftedfollowing a binding event. Since the refractive index of mostbiomaterial is close to that of glass (both low index around n=1.4),this binding can be modeled as a small increase in the glass layerthickness.

FIG. 5B shows the change in intensity due to a 1 nm increase in SiO₂thickness as a percent of the peak intensity recorded. A 1 nm increasein SiO₂ thickness corresponds, for instance, to about a 50 pg/mm² layerstreptavidin (50 kDa protein) binding to a microarray location 129. Thepercent change in intensity without using a layered substrate (relyingonly on increase absorption to affect the reflectivity) would benegligible and undetectable.

The SiO₂ on Si layered microarray 120 structure demonstrates theprinciple of (layered) substrate enhanced detection. More complicatedlayered structures may be used to further improve detection sensitivity.When choosing materials for the layered microarray 120 of thisinvention, it is important that the refractive index of the top layer beclosely matched to the index of the target molecules 124 to be detected.SiO₂ works well for many biomolecules including, for example, DNA, RNA,and protein. Also, the addition of target molecules 124 (e.g.,biomolecules) to the coating layer 122 significantly shifts thewavelength-reflectivity characteristic of the microarray 120 as a whole.

The direct reflectivity methodology combines the high throughputfeatures of a microarray with high sensitivity of laser detectionmethods in a manner that can be configured to provide real-time bindinginformation. Another advantage is that this can be accomplished usingrelatively inexpensive commercially available optical equipmentincluding a gray scale camera and simple tunable laser. Alternativeillumination sources 110 include, for example, a broad spectrum lightsource with a narrow tunable filter.

B. Split Beam Interferometry

The component set-up for split beam interferometry method is similar tothe direct reflectivity method described above in that the microarray120 is illuminated from above by an illumination light 212. In the splitbeam method, however, differs from the direct reflectivity method asshown by the schematic device 200 in FIG. 6. The illumination source210, in this case, is preferably a single wavelength laser, but a broadspectrum light with a narrow band-pass filter may also be used. Theillumination beam 212 is split by a beam splitter 240 into a sampleillumination beam 213 and a reference illumination beam 216 which aredirected onto the microarray 120 and the reference reflector 260,respectively. The sample illumination beam is reflected by themicroarray 120 as the sample image light 214 and the referenceillumination beam 216 is reflected by the reference reflector 260 as thereference image light 218. The sample image light 214 and the referenceimage light 218 are combined by the beam splitter 240 into the reflectedbeam 219 which is directed onto the photodetector 230. The referenceimage light 218 interferes with the sample image light 214 in thecombined reflected beam 219. As detailed further below, it is theinterference pattern that provides information about the apparent heightof the microarray 120.

During operation, the position of the reference reflector 260 is swept,perpendicular to the reference illumination beam 216, by a distance “d”.Interference between the reference image light 218 and the sample imagelight 214 changes as a function of the position of the referencereflector 260. As shown in FIGS. 7A-B, the photodetector 230 andassociated central processor create curves of reflective intensity vs.reference mirror position for each pixel. The curve shifts as theapparent height of the microarray changes. The calculation and analysismethodology is the same as for PSI {supra).

The standard split beam interferometry setup is capable of detectingmicroarray 120 surface heights with 1 nm accuracy or better for singlelayer, highly reflective surfaces. For low-index material such as SiO₂and biomaterial, standard interferometry perform poorly as a result ofthe low surface reflectivity. One simple solution is to use an equallylow reflectivity reference reflector 260 and turn up the source power tocompensate for the loss. The results can be improved even further,however, by using a layered substrate as described for the microarraysuseful in the standard illumination method. However, furthermodification of the layered substrate yields greater improvements. Forsplit beam interferometry, a thinner coating layer 122 and a thickerbase layer 121 are desirable. In one embodiment, the coating layer is270 nm of SiO₂ (n=1.4) on a thicker piece of Si for use with anillumination beam 112 having a wavelength (λ) of 1550 nm. Thus, thecoating layer 122 of SiO₂ has a thickness of λ/4 (i.e., “a quarter-wavelayer”). At this thickness, reflectivity around 1550 nm is minimized.Sensitivity of the interference pattern in the combined reflected beam219 to small changes in reference mirror position is, therefore,maximized. Table 1 shows the maximum change in intensity (as a percentof peak intensity) as a function of mirror position recorded during thedata collection.

Again, a quarter-wave layer of SiO₂ on Si is only one example of alayered substrate useful with the split beam method. More complicatedsubstrates may yield even greater sensitivities. The important featureof a desirable layered substrate is that the phase of the sample imagelight 214 change rapidly with the addition of biomaterial to top of thecoating layer 122 of closely matched index.

Another example a useful layered substrate for use in microarrays isSiO₂ on gold. Gold is highly reflective and so a highly reflective goldreference mirror should be used as well. The reflectivity of the samplewill remain high across different wavelengths, but the phase of thereflected light will change rapidly with the addition of a small amountof biomaterial when the apparent thickness of the SiO₂ layer isapproximately an odd integral number of wavelengths.

C. Modified Split Beam Interferometiy

The modified split beam interferometry set-up and methodology is verysimilar to the split beam method described above. The optical setup isthe same as that shown in FIG. 6 for split beam interferometry. Thedifference is that the illumination source 210 is a white light sourceinstead of a single wavelength laser. Again a reference reflector 260 isswept over a distance “d” with the difference being that interferenceusing a white light source will only be observed when the referencereflector 260 is about the same distance from the photodetector 230 asthe apparent surface of the microarray 120 (within the source coherencelength). The intensity of the interference is recorded at each pixel andcurves of intensity vs. reference reflector 260 position are againgenerated (FIGS. 8A-B). The reference reflector 260 position whereinterference is greatest indicates the apparent surface height at thatlocation. This method is normally accurate down to the nanometer scaleusing a highly reflecting sample reference mirror. Similar accuracy isattainable using low reflectivity samples with a low reflectivityreference mirror and turning up the source power. Additional sensitivityenhancements are gained from the use of a layered substrate in themicroarray. Table 1 shows the maximum change in intensity (as a percentof peak intensity) as a function of mirror position recorded during thedata collection using the same 270 nm SiO₂ on Si substrate. Theincreased sensitivity is because the layered substrate causes a phaseshift in the reflected light greater in some wavelength regionsdepending on the thicknesses of the layers chosen.

TABLE 1 Sensitivity Comparison of Layered Substrates Using VariousOptical Detection Methodologies Refer- Detec- ence tion Coat- Mate- Re-Sensitivity Me- Base ing rial flector 100*(Change in intensity) thod*Layer Layer Sensed Material (Peak Intensity Recorded) DRM Thick None 1nm — 0.00 SiO₂ SiO₂ DRM Thick 10 μm 1 nm — 0.47 Si SiO₂ SiO₂ SBI ThickNone 1 nm SiO₂ 0.41 SiO₂ SiO₂ SBI Thick 270 nm 1 nm SiO₂ 0.73 Si SiO₂SiO₂ MSBI Thick None 1 nm SiO₂ 0.32 SiO₂ SiO₂ MSBI Thick 270 nm 1 nmSiO₂ 0.65 Si SiO₂ SiO₂ *DRM = direct reflectivity method; SBI = splitbeam interferometry; MSBI = modified split beam inferometry.D. Microarray Detection

Microarray technology, including methods for making microarrays,procedures for conducting microarray experiments, and applications arewell known in the art [see, for example, Schena 2000]. The chemistry forthe attachment of capture molecules to SiO₂ substrates (i.e., thecoating layer) are also well known in the art. A significant advantageof the present invention over current microarray detection methodologiesis that the present techniques eliminate the need to detectably labelthe target molecules prior to performing the binding reaction on themicroarray. A second advantage of the present techniques describedherein is that a baseline height scan of the microarray may be performedprior to running the binding reaction with the target molecules so thatthe height of each detection location may be compared before and aftertarget molecule binding. This eliminates the need for height and/ordensity uniformity of the capture molecules across the entiremicroarray. It also serves to reduce interexperimental variability(e.g., that result from manufacturing defects) where replicatemicroarrays are compared.

An exemplary microarray useful in accordance with the principles of thisdisclosure may be created on the layered substrate by first cleaning thesubstrate surface with acetone, methanol, water, and N₂ gas, thenetching the surface with 10% NaOH for about 10 minutes. The surface isnext silanized and functionalized with an amino-silane (for instance:3-aminopropyl-triethoxysilane). Capture DNA is then be spotted atspatially distinct locations on the surface using a hollow pin (usuallydone robotically), using standard ink-jet printing technology [Hughes2001], or using standard photolithography techniques [Singh-Gasson1999]. The DNA is cross-linked (covalently bonded) to the surface byirradiation with UV light. The surface is finally rinsed before use.Alternative methods exist for making arrays including methods that usephotolithography.

The microarray is then scanned using any method in accordance with theprinciples of this invention to determine the initial height of eachlocation. Next, target DNA is introduced to the surface via a solution,rinsed, and dried. The array is scanned again using the same method andthe increase in height is recorded for each location. The change inheight is an index of the amount of target DNA bound to that location.

E. Alternative Substrate Designs

SiO₂ layered on Si, as described above, is the simple case of a layeredsubstrate that can be used in accordance with the principles of thisdisclosure. More complex layered substrates may also be used. The mostdesirable property for layered substrates is that the beam reflectedfrom the target molecule surface undergoes a maximum phase change for asmall change in the amount of material bound. A layered substrateconsisting of many semitransparent layers of different optical indicescan exhibit a very rapid change in both the intensity and phase of thereflected light for a small amount change in wavelength. Similarly, sucha structure may exhibit a very rapid change in phase for a small amountof material modifying the thickness of the top layer. For example, astack consisting of 270 nm of SiO₂, 340 nm of Si, 270 nm of SiO₂, 340 nmof Si, and 270 nm of SiO₂ on an Si substrate has high reflectivity forwavelengths of 1550 nm. If the top SiO₂ layer is slightly modified withbinding molecules, the reflectivity will remain high, but the phase willchange rapidly. This rapid phase change will give an enhanced signal ineither of the split beam interferometry methods described above.

The key is to create layered substrates that cause a rapid change in thephase of the reflecting light resulting from a small change in surfaceheight. Materials may be used other than Si and SiO₂. Alternativecoating layers that are useful in layered substrates include dielectricsubstances such as Si₃N₄.

In addition to the standard substrate configurations such as surfacecoating with capture molecules, either as a film or in spatiallydiscrete locations as a microarray, alternative configurations may beuseful depending upon the application. For example, in one alternativeembodiment, an additional discontinuous layer is formed on top of thecoating layer such that individual reaction wells are created. Thesereaction wells are desirably about 200 μm×200 μm×1 mm, but anyconvenient size may be used, depending upon the specific application andreaction conditions. These reaction wells may serve to contain thebonding reaction that fixes the capture molecules to the substratesurface, or the wells may be used to contain individual bindingreactions in spatially and/or chemically distinct environments. FIG. 12illustrates a layered substrate 500 having a base layer 121, a topcoating layer 122 a and an intermediate coating layer 122 b built inaccordance with the principles of this disclosure. The layered substrate500 further comprises reactions wells 575 formed by well walls 570. Eachreaction well 575 may contain the same or different reactants (e.g.,target molecules, capture molecules, biological/environmental samples),in a liquid medium, as the other reaction wells 575 on the same layeredsubstrate 500. The reaction wells 575 may be formed by layering apolymeric (e.g., PDMS) or other coating material (e.g., SiO₂) on top ofthe top coating layer 122 a. Alternatively, the reaction wells 575 maybe formed by etching or soft lithography such that the wells are “cut”into the top coating layer 122 a. For convenience, the layered substrate500 is shown in conjunction with the direct reflectivity measurementapparatus, but these substrates are also useful with either the splitbeam interferometry or the modified split beam inferometry methodologiesof this disclosure.

F. Real-Time Instrument Design

In order to perform real-time measurements so that binding kineticsbetween the target and capture molecules may be calculated, thesubstrate or microarray is desirably incorporated into a flow cell. Theflow cell should be at least semi-transparent and allow the delivery oftarget molecules to the capture molecules in an fluid environment.Suitable fluids include liquids (e.g., an aqueous solution) and gases(e.g., air or an inert gas). FIG. 10 is a schematic illustrating themanner in which a substrate 420 comprising capture molecules 123 (notshown) may be incorporated into a direct reflectivity device constructedin accordance with the principles of this disclosure. The flow cell 400is easily adapted for the split beam methodologies. When used in a flowcell 400 configuration, the height measurements indicating binding aredesirably be performed quickly and repeatedly to observe binding atvarious times during the pre-equilibrium state. This would enable theuser to extract kinetics information about the rate of the bindingreaction. If desired, the user may continue the measurements afterbinding equilibrium is reaching and change the fluid to one that lackstarget molecules in order to measure off-rate kinetics. The user mayalso introduce heat, pH change, electric field, or other physical orchemical alterations to further investigate binding kinetics.

EXAMPLE 1 Direct Reflectivity Method

FIG. 10 is a schematic of the optical set-up used for the followingexperiments.

The illumination source was a tunable wavelength laser (Anritsu MG9638A)that can step light between 1500 nm and 1580 nm in picometer increments,and was controlled using a GPIB card and central processor. The powerwas kept at about 0.1 mW. SMF-28 fiber optic carries the light from thelaser to the optical setup. The light exiting the fiber was collimatedby an objective and then beam expanded with two additional lenses. Therewas an aperture in the beam expander to spatial filter and pass thelowest order Gaussian mode exiting the fiber. The light then reached abeam splitter that directed part of the light onto the sample. The lightpath was as follows: the light from the laser was directed by a beamsplitter onto the sample; the light reflected from the sample and passedthrough the beam splitter, reaching the camera mounted above.

Imaging optics were included for imaging the sample surface onto thecamera. An objective was placed above the sample and below the beamsplitter. The light exiting the beam expander configuration wasconverging such that the objective lens re-collimated it when it passesthrough to the sample. Alternatively, the beam splitter may be placedclose to the sample and the objective above it. In this case, the beamexpander would produce a collimated beam. The illumination light thatreaches the sample in either case is collimated and the reflected lightis imaged. The lenses and beam splitter were AR coated for 1550 nm toavoid unwanted cavity effects that would produce a wavelengthdependency.

The camera used an InGaAs array (SU 128-1.7R; Sensors Unlimited). LabView software was used to control the laser and capture images from thecamera. An NI-1422 frame grabber card (National Instruments) was used tocapture the images.

A layered substrate “test sample” was prepared by oxidizing 5 microns ofSiO₂ on a 500 micron thick Si wafer piece. To mimic biomaterial bindingto the surface, 10 nm×100 micron strips of SiO₂ were patterned on thesubstrate surface. The index of SiO₂ is about n=1.4 which is verysimilar to the index of DNA and many proteins.

The illumination light wavelength was stepped from 1500 nm to 1580 nm in0.2 nm increments and an image was recorded at each step. First, a darkframe is captured with the lens cap on the camera. This dark frame wassubtracted from all future measurements taken by the camera. A referencesample was used to characterize any wavelength dependence of the system.Then, the reference sample was replaced with the “test sample” and themeasurement procedure was repeated.

The intensity vs. wavelength curve for each pixel was obtained anddivided by the characteristic determined by the reference scan. Theresulting curves were fit in a least squares sense to a model similar tothe Matlab code entitled Method A included in the software package(Mathworks, Inc., Ver. 7.0.1). This curve-fitting model is based on thescattering matrix method for determining the reflectivity of thin filmsas described in chapter 5 of “Optical Waves in Layers Media” [Yeh 1988].Alternatively, the curves may be fit to a simple sine wave as anapproximation of the true reflectivity vs. wavelength curve. Anotheralternative is to take the Fourier transform of a collected curve andobserve a phase increase which indicates a shift in the curve whichindicates a change in surface height. However, results using thescattering matrix model are most accurate because the model accounts formultiple reflections within the semitransparent layer. The scatteringmatrix method works equally well for calculating the multiplereflections in more complicated structures with more layers.

FIGS. 11A-C provide the results of this experiment. The relative layerthickness across the sample surface gives the change in height acrossthe surface may be plotted as a 3D mesh or as a 2D image where theintensity of each pixel indicates its height. FIG. 11A is a graph ofwavelength vs. intensity for two different pixels. The actual data andthe fitted curve are shown. The lower curve represents a pixelcorresponding to a location that has a greater height than the locationcorresponding to the upper curve. FIG. 11B is a 3D mesh graph showingthe height measured at each pixel. FIG. 11C is a 2D image showing theintensity at each pixel in the image which represents the calculatedheight at the corresponding location.

This experiment has been successfully repeated with a visible CCD camera(Rolera from QImaging) and a tunable laser (New Focus Velocity) with acenter wavelength of about 770 nm. Measurements of the 10 nm high stripswere repeated with better than 1 nm repeatability.

REFERENCES

-   (1) Brockman, et al. (2000). Surface Plasmon Resonance Imaging    Measurements of Ultrathin Organic Films. Annu. Rev. Phys. Chem. 51,    pp. 41-63.-   (2) Hughes, T. R., et al. (2001). Expression profiling using    microarrays fabricated by an ink-jet oligonucleotide synthesizer.    Nature Biotechnology 19, pp. 342-7.-   (3) Jenison, et al. (2001). Interference-based detection of nucleic    acid targets on optically coated silicon. Nature Biotechnology, Vol.    19 pp. 62-65.-   (4) Lin, et al. (2002). A label-free optical technique for detecting    small molecule interactions.-   Biosensors & Bioelectronics 17, pp. 827-834.-   (5) Lukosz (1991). Principles and sensitivities of integrated    optical and surface plasmon sensors for direct affinity sensing and    immunosensing. Biosensors & Bioelectronics 6, pp. 215-225.-   (6) Moiseev, et al. (2004). Spectral Self Interference Fluorescence    Microscopy. J. Appl. Phys, Vol. 96, No. 7.-   (7) Moiseev, et al. (2006). DNA conformation on surfaces measured by    fluorescence self-inteference. PNAS Feb. 21, 2006. Vol. 103, No. 8,    pp. 2623-2628.-   (8) Piehler, et al. (1996). Affinity Detection of Low Molecular    Weight Analytes. Anal. Chem. 68, pp. 139-143.-   (9) Schena (2000). Microarray Biochip Technology. Eaton. 2000. ISBN:    1-881299-37-6.-   (10) Singh-Gasson S, et al. (1999). Maskless fabrication of    light-directed oligonucleotide microarrays using a digital    micromirror array. Nature Biotechnology 17 No. 10, pp. 974-978.-   (11) Unlu, et al. (2004). Spectroscopy of Fluorescence for Vertical    Sectioning. United States Patent Publication 2004/0036884.-   (12) Unlu, et al. Resonant Cavity Imaging Biosensor.    PCT/US2004/008558.-   (13) Yeh (1988). Optical Waves in Layered Media. Wiley. ISBN:    0471828661.-   (14) Zhang, et al. (2004). Micromechanical measurement of membrane    receptor binding for label-free drug discovery. Biosensors and    Bioelectronics. 19, pp. 1473-1478.

Other Embodiments

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it will be readily apparent to those of ordinary skill inthe art in light of the teachings of this invention that certain changesand modifications may be made thereto without departing from the spiritor scope of the appended claims.

What is claimed is:
 1. An optical detection apparatus comprising: (i) areflective substrate having an uppermost surface, the reflectivesubstrate comprising capture molecules adsorbed or covalently bound tothe uppermost surface; (ii) a tunable light source positioned above thereflective substrate, the tunable light source comprising anillumination beam having a tuning range of light comprising varyingwavelengths, the illumination beam being directed substantiallyperpendicular to the uppermost surface of the substrate; and (iii) aphotodetector array operably linked to a central processor configured tomeasure an intensity of light reflected from the substrate.
 2. Theapparatus of claim 1, wherein the light source is selected from thegroup consisting of a tunable laser and a broad spectrum light sourceoperably linked to a tunable filter.
 3. The apparatus of claim 1,wherein the substrate is a layered substrate comprising a base layer, atleast one coating layer having a refractive index different from therefractive index of the base layer, and a plurality of spatiallydistinct binding locations, wherein each of the binding locationscomprises capture molecules bound to the topmost coating layer.
 4. Theapparatus of claim 1, wherein the photodetector array is selected fromthe group consisting of a CCD camera and an InGaAs array.
 5. Theapparatus of claim 1, wherein the light source is tunable over awavelength range of at least 5 nm.
 6. The apparatus of claim 1, whereinthe light source is tunable within a wavelength range comprising about1450 nm to about 1650 nm.
 7. The apparatus of claim 1, wherein thecentral processor is capable of calculating reflectivity curves ofillumination intensity as a function of illumination wavelength forsubstantially every pixel of the photodetector array.
 8. An opticaldetection apparatus comprising: (i) a reflective substrate having anuppermost surface, the reflective substrate comprising capture moleculesadsorbed or covalently bound to the uppermost surface; (ii) a tunablelight source positioned above the reflective substrate, the tunablelight source comprising an illumination beam having a tuning range oflight comprising varying wavelengths, the illumination beam beingdirected substantially perpendicular to the uppermost surface of thesubstrate; (iii) a movable reference reflector having substantially thesame refractive index as the reflective substrate; (iv) a first beamsplitter capable of splitting the illumination beam into a sampleillumination beam and a reference illumination beam, the beam splitterfurther capable of directing the sample illumination beam onto areflective substrate and directing the reference illumination beam ontoa reference reflector; (v) a second beam splitter capable of combiningthe light reflected from the reflective substrate with the lightreflected from the reference reflector into an imaged light beam; and(vi) a photodetector array positioned to measure the intensity of theimaged light beam operably linked to a central processor configured tocapture images produced by the photodetector array.
 9. The apparatus ofclaim 8, wherein the illumination beam is substantially a singlewavelength.
 10. The apparatus of claim 8, wherein the light source isselected from the group consisting of a tunable laser and a broadspectrum light source operably linked to a tunable filter.
 11. Theapparatus of claim 8, wherein the illumination beam comprises multiplewavelengths.
 12. The apparatus of claim 11, wherein the illuminationbeam is white light.
 13. The apparatus of claim 8, wherein the substrateis a layered substrate comprising a base layer, at least one coatinglayer having a refractive index different from the refractive index ofthe base layer, and a plurality of spatially distinct binding locations,wherein each of the binding locations comprises capture molecules boundto the topmost coating layer.
 14. The apparatus of claim 8, wherein thephotodetector array is selected from the group consisting of a CCDcamera and an InGaAs array.
 15. The apparatus of claim 8, wherein theillumination beam has a wavelength of between about 1450 nm to about1650 nm.
 16. The apparatus of claim 8, wherein the central processor iscapable of calculating reflectivity curves of illumination intensity asa function of a displacement position of the reference reflector forsubstantially every pixel of the photodetector array.